Although this is still being debated, I would say this is still more than an opinion. Try this thought experiment:

You want to get as strong as possible. It is well known that the body adapts to the demands imposed upon it, so to get strong you know you must train with heavy resistance in one form or another. You cannot lift a heavier weight as many times as a lighter weight in one bout, but strength is a function of both the nervous system and your muscular capacity via hypertrophy. Both of these things are highly correlated with repeated exposure with deliberate practice as long as there is sufficient time to recover between bouts. The logical conclusion for strength training is therefore heavier resistance with more rest between more sets.

It's very much up for debate what an ideal program might be, but 5 rep sets at around 70-85% of your 1RM with a total set volume per exercise between 15-30 is something of an average of many of the most historically successful approaches.

Which is okay, because it's not a program for gaining strength, it's a program for gaining muscle - thus the emphasis on time under tension, etc.

---

I don't disagree with what Gypsy Jazz posted, although I'd take the "5 rep sets" part of it, put it in big bold font and drag it up to the top of the post, because I think it's the most important part. If you want to gain maximal strength, you need to move relatively heavy weights; once you're doing that, more volume (sets) can help you make more progress, work through plateaus, etc.

But if you're not using heavy enough weights - say, if you're doing sets of 10+ reps - it doesn't matter how many sets and reps you do; you're going to be disappointed with your maximal strength gains.

Understood. You mean that there can be a big difference between exercising for hypertrophy and exercising for strength. Right now I'm doing weights that I can do 6-10 reps with for upper body and 12 - 20 for lower body. When I hit the high part of the rep range I increase the weight.